7,199 research outputs found

### Relativistic non-equilibrium thermodynamics revisited

Relativistic irreversible thermodynamics is reformulated following the
conventional approach proposed by Meixner in the non-relativistic case. Clear
separation between mechanical and non-mechanical energy fluxes is made. The
resulting equations for the entropy production and the local internal energy
have the same structure as the non-relativistic ones. Assuming linear
constitutive laws, it is shown that consistency is obtained both with the laws
of thermodynamics and causality.Comment: 11 pages, no figure

### Traversable wormholes: minimum violation of null energy condition revisited

It was argued in literature that traversable wormholes can exist with
arbitrarily small violation of null energy conditions. We show that if the
amount of exotic material near the wormhole throat tends to zero, either this
leads to a horn instead of a wormhole or the throat approaches the horizon in
such a way that infnitely large stresses develop on the throat.Comment: 12 pages. To appear in PR

### `Mass without mass' from thin shells in Gauss-Bonnet gravity

Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity.
There is the well known junction condition for the singular part of the stress
tensor intrinsic to the shell, which we also prove to be well defined. There
are also equations relating the geometry of the shell (jump and average of the
extrinsic curvature as well as the intrinsic curvature) to the non-singular
components of the bulk stress tensor on the sides of the thin shell.
The equations are applied to spherically symmetric thin shells in vacuum. The
shells are part of the vacuum, they carry no energy tensor. We classify these
solutions of `thin shells of nothingness' in the pure Gauss-Bonnet theory.
There are three types of solutions, with one, zero or two asymptotic regions
respectively. The third kind of solution are wormholes. Although vacuum
solutions, they have the appearance of mass in the asymptotic regions. It is
striking that in this theory, exotic matter is not needed in order for
wormholes to exist- they can exist even with no matter.Comment: 13 pages, RevTex, 8 figures. Version 2: includes discussion on the
well-defined thin shell limit. Version 3: typos fixed, a reference added,
accepted for publication in Phys. Rev.

### Junction conditions in General Relativity with spin sources

The junction conditions for General Relativity in the presence of domain
walls with intrinsic spin are derived in three and higher dimensions. A stress
tensor and a spin current can be defined just by requiring the existence of a
well defined volume element instead of an induced metric, so as to allow for
generic torsion sources. In general, when the torsion is localized on the
domain wall, it is necessary to relax the continuity of the tangential
components of the vielbein. In fact it is found that the spin current is
proportional to the jump in the vielbein and the stress-energy tensor is
proportional to the jump in the spin connection. The consistency of the
junction conditions implies a constraint between the direction of flow of
energy and the orientation of the spin. As an application, we derive the
circularly symmetric solutions for both the rotating string with tension and
the spinning dust string in three dimensions. The rotating string with tension
generates a rotating truncated cone outside and a flat space-time with
inevitable frame dragging inside. In the case of a string made of spinning
dust, in opposition to the previous case no frame dragging is present inside,
so that in this sense, the dragging effect can be "shielded" by considering
spinning instead of rotating sources. Both solutions are consistently lifted as
cylinders in the four-dimensional case.Comment: 24 pages, no figures, CECS style. References added and misprints
corrected. Published Versio

### Higher-Dimensional Bulk Wormholes and their Manifestations in Brane Worlds

There is nothing to prevent a higher-dimensional anti-de Sitter bulk
spacetime from containing various other branes in addition to hosting our
universe, presumed to be a positive-tension 3-brane. In particular, it could
contain closed, microscopic branes that form the boundary surfaces of void
bubbles and thus violate the null energy condition in the bulk. The possible
existence of such micro branes can be investigated by considering the
properties of the ground state of a pseudo-Wheeler-DeWitt equation describing
brane quantum dynamics in minisuperspace. If they exist, a concentration of
these micro branes could act as a fluid of exotic matter able to support
macroscopic wormholes connecting otherwise distant regions of the bulk. Were
the brane constituting our universe to expand into a region of the bulk
containing such higher-dimensional macroscopic wormholes, they would likely
manifest themselves in our brane as wormholes of normal dimensionality, whose
spontaneous appearance and general dynamics would seem inexplicably peculiar.
This encounter could also result in the formation of baby universes of a
particular type.Comment: 21 pages, 1 figur

### Low energy effective gravitational equations on a Gauss-Bonnet brane

We present effective gravitational equations at low energies in a
$Z_2$-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based
on the geometrical projection approach, and we solve iteratively the bulk
geometry using the gradient expansion scheme. Although the original field
equations are quite complicated due to the presence of the Gauss-Bonnet term,
our final result clearly has the form of the Einstein equations plus correction
terms, which is simple enough to handle. As an application, we consider
homogeneous and isotropic cosmology on the brane. We also comment on the
holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.Comment: 10 pages, v2: minor clarification

### High-energy effective theory for orbifold branes

We derive an effective theory on the orbifold branes of the Randall-Sundrum 1
(RS1) braneworld scenario in the presence of a bulk brane. We concentrate on
the regime where the three branes are close and consider a scenario where the
bulk brane collides with one of the orbifold branes. This theory allows us to
understand the corrections to a low-energy approach due to the presence of
higher velocity terms, coming from the Kaluza-Klein modes. We consider the
evolution of gravitational waves on a cosmological background and find that,
within the large velocity limit, the boundary branes recover a purely
four-dimensional behavior.Comment: 4 pages, accepted for publication in Phys. Rev.

### Incompressible fluid inside an astrophysical black hole?

It is argued that under natural hypothesis the Fermions inside a black hole
formed after the collapse of a neutron star could form a non compressible fluid
(well before reaching the Planck scale) leading to some features of integer
Quantum Hall Effect. The relations with black hole entropy are analyzed.
Insights coming from Quantum Hall Effect are used to analyze the coupling with
Einstein equations. Connections with some cosmological scenarios and with
higher dimensional Quantum Hall Effect are shortly pointed out.Comment: 30 pages, 2 figures. Accepted for publication on Physical Review D:
references added, typos corrected, test polishe

### String Supported Wormhole Spacetimes and Causality Violations

We construct a static axisymmetric wormhole from the gravitational field of
two Schwarzschild particles which are kept in equilibrium by strings (ropes)
extending to infinity. The wormhole is obtained by matching two
three-dimensional timelike surfaces surrounding each of the particles and thus
spacetime becomes non-simply connected. Although the matching will not be exact
in general it is possible to make the error arbitrarily small by assuming that
the distance between the particles is much larger than the radius of the
wormhole mouths. Whenever the masses of the two wormhole mouths are different,
causality violating effects will occur.Comment: 12 pages, LaTeX, 1 figur

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